There are a number of different devices available that indicate when a predetermined or rated temperature has been exceeded. These include the following: (1) non-reversible labels with an indicator dot that changes color, e.g., turns black, at a rated temperature; (2) temperature indicating tabs, crayons, lacquers and pellets that indicate a temperature increase by melting; (3) irreversible labels and monitors that change color (e.g., turn black) at the temperature rating and also register the temperature history of the area; (4) melting point standards that melt at precise temperatures; and (5) reversible labels and liquid crystal thermometers that behave like conventional thermometers to provide the present temperature of an object.
A serious disadvantage of most of these devices is that the temperature setting at which the temperature indicator changes color is above room temperature, i.e., the devices do not operate below room temperature. The explanation for this temperature limitation may be that such devices cannot be manufactured for technical reasons or that it is considered impractical to do so because of the inconvenience of having to maintain the devices below room temperature, e.g., in a refrigerator or freezer until ready to use. Of course, thermometers will operate below room temperature but they do not provide a permanent record of a maximum temperature being exceeded and thus will not alert a user after the fact.
In addition the devices discussed above, there are a number of other devices which provide temperature indications, including devices which operate below room temperature. Examples of these include the devices disclosed in the following U.S. Pat. Nos.: 4,145,918 (Couch et al); 4,134,359 (Redpath); 4,120,818 (Swindells et al); 4,038,936 (Kessler); 3,702,077 (Szabo); 3,545,400 (Smith); 3,414,415 (Broad, Jr.); 2,896,568 (Pryor et al); 2,560,537 (Andersen); 2,823,131 (Power).
Briefly considering these patents, the Couch et al patent discloses a freeze and thaw indicator which relies on the expansion characteristics of a liquid (water) when the liquid changes from the liquid state to the solid state. The indicator includes an ampul which breaks when the water freezes. During a subsequent thawing, water flows on indicator paper to cause ink thereon to wick up. The Redpath patent discloses a temperature indicator device wherein melting of a frozen indicator will result in the indicator flowing through a barrier and discoloring a material to indicate that proper storage conditions have not been maintained. The Swindells et al patent discloses an irreversible warmup indicator wherein melting of a liquid after freezing thereof causes the liquid to escape through an orifice to contact indicating papers and provide dying of the paper. The Kessler patent discloses a condition change indicator wherein two communicating chambers contain distinctly different substances. After arming, either the frozen state of the fluid in the lower chamber or an unmelted plug between the chambers permits mixing of the substances until the liquid change to be indicated has taken place. The remaining patents disclose other devices for indicating defrosting, thawing or other temperature changes.